Figure out showing errors onBlur

It's a pretty common use case to want to show errors after blur. see `blur` branch for some initial experiments.

Start by comparing the current validation behavior with the `blur` branch, which contains initial experiments for this request. No files or tests are named, so first locate the form validation entry points and existing error-display tests. Done means the intended onBlur error behavior is defined, implemented, and covered by tests.

Written by the indexing model from the issue text.